The introduction and integration of distributed energy resources (DER) into the electric power system (EPS, or “grid”) has become a priority in the modern energy era. Distributed energy resources include resources that provide generation (such as photovoltaic, fuel cell, wind, diesel, and natural gas generators), load (such as buildings, homes, and electric vehicles), or storage (such as batteries, flywheels, supercapacitors, and pumped hydroelectric). In particular, the integration of renewable energy sources and electric vehicles onto the grid has many important economic and environmental benefits. Storage is considered as a “missing piece” of the distribution system, performing functions such as peak shaving/valley filling, Volt/VAR optimization, capacity relief, power quality management, buffering the intermittency and variability of supply (e.g. power generation from renewable sources) and demand (e.g. electric vehicle charging or large thermostatic loads), providing backup power, and participating in power system ancillary services.
A microgrid can be defined as a group of interconnected loads and distributed energy resources with clearly defined electrical boundaries that acts as a single controllable entity with respect to the grid and can connect and disconnect from the grid to enable it to operate in both grid-connected or island mode, according to the United States Department of Energy.
Today's microgrid controllers do not generally take into account the overall configuration and operation of the power grid. Their primary purpose is to optimize the operation within the microgrid itself under a fixed electrical boundary without consideration for the external grid. The only consideration of the existing microgrid controllers is not to exceed some fixed limits for current, voltage, and frequency. Some of microgrid controllers are able to accommodate work scheduling, on/off device switching, and outage management. Energy management is usually based on selective activation of a multiplicity of power generation equipment over a predetermined distribution and/or storage to supply a microgrid of electrical power, and automatic, selective disconnect any of power generators from providing power supply to the microgrid. The demand side is managed through conservation and demand response programs and premise (e.g. building, home) management and automation systems. Both of these approaches have little or no significance for distribution system operations. Microgrid controllers typically do not interoperate with the distribution system's DMS, and little value can be attained for the local distribution companies.
Various microgrid controller solutions have been provided in the prior art. The microgrid control system described in US Patent Application No. 20140252855 to Tohru Watanabe, et al. is capable of controlling multiple facilities according to characteristics of the facilities in order to achieve economic efficiency, environmental friendliness, and continued operability. The microgrid control system for controlling the operations of the multiple power facilities is provided with a power supply activation/suspension planning unit that has a prediction unit for predicting outputs or loads of power supply facilities or load facilities and a prediction unit for predicting prediction errors contained therein. It is also provided with an economical load allocation unit that determines command values related to the distribution of loads to be borne by currently running power supply facilities.
In U.S. Patent Application No. 20140297051 to Huaguang Zhang, et al., an energy resource-grid-load automatic control system is presented. The energy resource-grid-load automatic control system comprises a distributed renewable energy power generation module, a distributed renewable energy inverter module, a conventional power generation module, a user load module, a bidirectional grid-connected control module, a distributed renewable energy intelligent optimizing power generation control module, an energy storage module, an intelligent energy storage unit adjuster and a storage battery pack. According to the authors, this controller guarantees the stability and the high energy utilization of the power generation system and effectively solves the problem of non-uniform frequency of use of storage batteries to unify the service life of the storage battery pack.
U.S. Patent Application No. 20130041516 to Uwe Rockenfeller, et al. presents a microgrid controller that may control the generation, distribution, storage and use of electrical power on a microgrid. Embodiments of a microgrid controller may include inputs for different types of power (e.g. AC and DC) or power sources (e.g. wind and solar), an input for utility grid power, electrical equipment for conditioning the electrical power received from the multiple sources (e.g. rectifiers and inverters), outputs to multiple types of loads (e.g. three-phase AC and single-phase AC) and control circuitry designed to control the generation, storage, distribution and usage of electrical power on the microgrid. Embodiments of microgrid systems may include multiple types of electrical generation sources (e.g. wind, solar, electromechanical and fuel cell), multiple types of electrical loads (e.g. inductive and resistive), electrical storage units (e.g. batteries) and a microgrid controller.
In U.S. Patent Application No. 20140300182 to John L. Creed, et al., methods and control apparatus are presented for controlling supply of electrical power to a micro-grid power system, in which a master controller automatically rebalances the micro-grid by activating and deactivating individual power supplies to preferentially activate non-fuel consuming power supplies and deactivate fuel consuming power supplies so as to minimize fuel consumption for the micro-grid power system.
In U.S. Pat. No. 8,682,495 to Michael A. Carralero, et al., a method, apparatus, and computer program product is provided for configuring a microgrid. A first configuration of the microgrid having a set of microgrid elements is initialized. An address for each element in the set of microgrid elements of the microgrid is verified. In response to receiving status data from the set of microgrid elements connected in a peer-to-peer network indicating a reconfiguration of the microgrid, the set of microgrid elements is re-aligned to form a second grid configuration. A second configuration of the microgrid is then executed.
In International PCT Patent Application No. WO2013015773 to Hussam Alatrash, et al., a method, hierarchy, and control architecture for supervisory control of microgrids and their respective energy resources may be provided with the aim of building safe, reliable, and scalable microgrids. Furthermore, the hierarchy and control architecture may be aimed at supporting a host electrical power system stability and while waiving interconnection requirements that challenge system stability.
Although they provide specific microgrid functions, the above-referenced solutions fail to address the issue of the overall power grid optimization. They are generally not designed to contribute reactive power to the grid in order to minimize losses or to improve voltage profile along the distribution feeder. They do not adequately address the issue of significant power swings due to large loads (one-, two- or three-phase) being randomly connected to the microgrid.